LONDON (AP) — Queen Elizabeth II attended a service at Westminster Abbey on Thursday, the eve of the centenary of the Battle of the Somme, one of the deadliest chapters of World War I.

The 90-year-old British monarch laid a wreath of roses on the grave of the Unknown Warrior inside the ancient abbey in London before a bugler sounded the Last Post, a tribute to the fallen. An honor guard of soldiers and civilians will hold an overnight vigil at the grave, ending just before 7:30 a.m., the time British troops were sent into battle on July 1, 1916.

The British and French offensive against German forces ground on for 141 days in 1916, leaving more than a million dead or wounded.

Prince William, his wife Kate and Prince Harry attended a vigil Thursday evening at the Thiepval Memorial in northern France, where 70,000 British and Commonwealth soldiers are commemorated.

On Friday, the young royals will join Prime Minister David Cameron, French President Francois Hollande and other dignitaries from the combatant nations at a service in France.

Cameron said it was "an opportunity to think about the impact of the devastation felt by communities across all of the nations involved, which left mothers without sons, wives without husbands and children without fathers."

"But today is also a chance to stand as friends with the representatives of all the countries who are here today," said Cameron, whose country voted last week to leave the European Union, an international alliance set up in the aftermath of another conflict, World War II.

The vote has cost Cameron his job and a leadership race is now underway in Britain's Conservative Party to replace him.